In a conventional thermal transfer printer, an ink ribbon and printing paper are sandwiched between a thermal head and a platen roller to melt ink on the ink ribbon with heat generated by the thermal head. Then, by transferring and fixing the molten ink onto the printing paper, the thermal transfer can print predetermined information on the printing paper.
An example of an existing thermal transfer printer will be described in detail with reference to FIGS. 4A-4B.
FIG. 4A is a schematic side view of a thermal transfer printer 1. The thermal transfer printer 1 includes a housing 2, a printing paper supply unit 3, a printing unit 4, and a printing paper cutting unit 5. A discharge port 6 is formed in the housing 2 downstream from the printing paper cutting unit 5.
The printing paper supply unit 3 has a supply shaft 7, and a rolled printing paper 8 is held by the supply shaft 7. The printing unit 4 includes a thermal head 9, a platen roller 10 that is located to oppose the thermal head 9, an ink ribbon supply unit 11, and an ink ribbon take-up unit 12. An ink ribbon 13 is guided from the ink ribbon supply unit 11 to the ink ribbon take-up unit 12 by guide rollers 14 to stretch along a lower portion of the thermal head 9.
The printing paper 8 that is supplied from the printing paper supply unit 3 is transported by a transport roller 15 toward the printing unit 4 located downstream from the printing paper supply unit 3. The transported printing paper 8 is sandwiched, together with the ink ribbon 13, between the thermal head 9 and the platen roller 10. At this point, ink on the ink ribbon 13 melts with heat generated in the thermal head 9, and predetermined information is printed on the printing paper 8. After printing, the printing paper 8 is cut by the printing paper cutting unit 5 in a desired length and discharged to the outside of the thermal transfer printer 1 through the discharge port 6.
FIG. 4B is an enlarged sectional view of a portion of the printing paper 8 enclosed by a circled dotted line 4B in FIG. 4A. As illustrated in FIG. 4B, the printing paper 8 includes a base material 16 that is coated with an adhesive 17. The base material 16 is temporarily affixed to a board 19 via a silicone layer 18 that serves as a release agent. As ink is transferred and fixed onto a surface of the base material 16, the predetermined information is displayed thereon.
Subsequently, the thermal head 9 to be used in the thermal transfer printer 1 will be described with reference to FIG. 5.
FIG. 5 is a rear view of the thermal head 9 as viewed from the side of the platen roller 10. A direction shown by an arrow in FIG. 5 indicates a transport direction of the printing paper 8 from an upstream side toward a downstream side.
The thermal head 9 includes a head body 20 that is configured of a heatsink, a heat generator 21, a connector unit 22 which transmits a signal to control an amount of heat in the heat generator 21 through a control unit in a body (printer body) of the thermal transfer printer 1, and an engaging unit 23 which supports the thermal head 9 onto an unillustrated bracket of the body.
The heat generator 21 includes a plurality of heating elements arrayed intermittently extending parallel in a widthwise direction of the thermal head 9. The heating elements emit heat to melt the ink on the ink ribbon 13. The predetermined information is printed as the molten ink is transferred and fixed onto the printing paper 8.
In the thermal transfer printer 1 configured as described above, circumstances that lead to printing failure have been observed. The circumstances include the ink ribbon 13 sagging, being wrinkled, or traveling obliquely. As a method of retaining constant tension of the ink ribbon 13 in order to prevent wrinkles or the like from being generated in the ink ribbon 13, a method exists in which a speed at which the ink ribbon 13 is taken up is controlled in an unillustrated control unit such that the speed at which the ink ribbon 13 is taken up exceeds a speed at which printing paper is transported, or a method in which a guide member is provided to guide the ink ribbon 13 at a position where the ink ribbon 13 is separated from the printing paper 8 in the thermal head 9 and redirected toward the ink ribbon take-up unit 12 (see, for example, PTL 1).
However, in the thermal transfer printer 1 having the above-described configuration, when, for example, black and white are reversed in printing or printing density is increased, thermal energy is concentrated at a part of the ink ribbon 13. Then, a wave-like warp is generated partially in the ink ribbon 13 due to the heat. There is a possibility that wrinkles are generated due to the warp and uniform tension is not given to the ink ribbon 13, which results in printing failure or oblique traveling of the ink ribbon 13.